1. Technical Field
This invention relates generally to an improved strategy for clearing paper jams in document production apparatus.
2. Background Art
Document production apparatus, which includes copiers and printers, commonly employ several shutdown procedures depending on the cause of the shutdown. For example, a normal shutdown is invoked at the end of a successful run, and is started as soon as the last image of the run has been written to the apparatus. On the otherhand, a so called hard shutdown is invoked when a catastrophic condition occurs which may damage the apparatus; and receiver sheet movement is stopped; at least upstream of the problem. A soft shutdown is invoked when a problem occurs which allows all successfully fed receiver sheets to continue processing until finished.
Many conventional types of document production apparatus employ jam recovery procedures which maintain the integrity of the run in progress while receiver sheet jams are cleared. The procedures allow the production run to restart at the position where the trouble was encountered.
Often, attempts are made to design the system to minimize the operator involvement in clearing the jam. For example, U.S. Pat. No. 4,878,428, issued to Watarai on Nov. 7, 1989, and Japanese 56-137367 of Oct. 27, 1981, disclose document copiers wherein a jam in the receiver sheet feeder section stops only the transport in that section, allowing continued operation of the sheet transport section down-stream of the sheet feeder section until all sheets which were fed from the paper supply before the jammed sheet was fed have been ejected. In another example of the prior art, U.S. Pat. No. 4,231,567 provides for sheets downstream of the jam to continue to the exit tray, and sheets upstream of the jam to proceed to a cluster area for easier clearance.
Recent document production concepts involve an electronic front end and capability for two-sided (duplex) document productions. Such apparatus would handle jobs with any sequence of page types, e.g., one-sided (simplex) or two-sided. The corresponding variations in receiver sheet path impose constraints on exposure timing. These constraints, along with the need for collated output, efficient frame utilization, and high throughput rate lead to schemes for judicious scheduling of exposures according to the page type sequence of a particular job.
An efficient schedule's exposure order is almost always different from the desired (collated) output page order. With a multi-page electronic image buffer on the electronic front end, this unnatural exposure order should present little difficulty. If the sequence of page types is known at the start of the job, a customized exposure schedule can be computed to complete the job in minimum time.
For example, commonly assigned, co-pending U.S. patent application Ser. No. 07/430,037 discloses a reproduction apparatus having a duplex turn-around path requiring a plurality of skip-frames between first and second sides of a duplex page. This suggests that the first side image may be scheduled for exposure and transfer even before a simplex page which is to precede the duplex page in the collated document set has been exposed. By the time the duplex receiver sheet has been transported around the duplex path and returned for the second side, the simplex page has been produced. This situation complicates jam shutdown procedures since a jammed sheet may be preceded by a sheet which will actually follow the jammed sheet in the finished document set.